1. Field of the Invention
The present invention relates to a fixation unit and an image forming apparatus.
2. Description of Related Art
In image forming apparatuses which form toner images on paper, one of widely-known fixation units nips paper, which supports an unfixed toner image, between rotary bodies such as facing rollers and a belt, and conveys the paper with pressure and heat applied thereto for fixation of the toner image on the paper.
If paper is nipped and subjected to the heat fixing process by such a fixation unit, the paper sometimes slips between the rotary bodies because of melting toner when the toner image formed on the paper includes a large amount of toner. For example, the degrees of slip of paper are different between a region of paper including toner (including much toner) and a region thereof including no toner (including a little toner). Accordingly, driving torque that can be transmitted from the driving rotary body to the driven rotary body is varied, thus causing failure called paper wrinkling that paper wrinkles.
There is a technique to prevent a pressure belt of the fixation unit from being reduced in speed by slip of paper (for example, see Japanese Patent Application Laid-Open Publication No. 02-222980) in the following manner. The pressure belt of the fixation unit is provided so as to be driven and rotated in the direction that the paper is conveyed through a one-way clutch, which allows free movement in the paper conveyance direction. Moreover, the circumferential speed of the pressure belt is set to not higher than the circumferential speed of the heating roller, thus stabilize the paper conveyance by the pressure belt and the heating roller.
By another one of known techniques (for example, see Japanese Patent Application Laid-open Publication No. 2006-71727), a driving torque giving means, which gives a photoreceptor drum a driving torque of a driving motor for driving a pair of fixing rollers, includes a torque limiter for stably giving the driving torque.
However, in the case of the aforementioned patent literature (Japanese Patent Application Laid-Open Publication No. 02-222980), because of use of the one-way clutch, auxiliary drive functions only when the speed of the pressure belt is reduced to a predetermined value or less, and on/off of the auxiliary drive depends on the average belt speed in the nip region. On the other hand, occurrence of the paper wrinkling is largely due to the presence or absence of toner images and the density thereof in the longitudinal direction of the nip region. Accordingly, the paper wrinkling occurs in some cases if the shear force that the paper receives from the belt varies in the longitudinal direction of the nip region even when the belt does not slip on average. Accordingly, this technique also cannot prevent the paper wrinkling.
In the case of the aforementioned patent literature (Japanese Patent Application Laid-Open Publication No. 2006-71727), if the circumferential speed of the belt is set lower enough than the circumferential speed of the roller when auxiliary driving force is given by the torque limiter, the auxiliary torque is always fixed to the torque limiting value, thus preventing the paper wrinkling to a certain extent. However, in the torque limiter which is in operation to limit the torque, slip is always caused. Accordingly, in the fixing process with large driving force, the aforementioned technique is impractical in the light of the loss of driving force and the durability of the torque limiter.